In recent years, in crude oil and natural gas pipelines, increase of the internal pressure for the purpose of improving the transport efficiency and reduction of the outer diameter and weight of line pipe for the purpose of improving the efficiency of on-site installation have been demanded. High strength steel pipe with a tensile strength in the circumferential direction or 760 MPa to less than 900 MPa, corresponding to the X100 class of the API standard, has been developed (for example, Japanese Patent Publication (A) No. 11-140580 and Japanese Patent Publication (A) No. 2003-293078).
Further, in pipelines, a ductile crack occurring in the base material of the steel pipe may propagate in the pipe axial direction at a high speed of 100 m/s or more over a long distance of 100 meters to several km. An arrest characteristic is therefore demanded. The “arrest characteristic” is the characteristic of arresting the propagation of cracks. It is classified into the characteristic of arresting the propagation of an embrittlement crack through the base material, that is, the embrittlement fracture resistance characteristic, and the characteristic of arresting the propagation of a ductile crack through the base material, that is, the ductile fracture characteristic. Among these, for the embrittlement fracture resistance characteristic, a structural steel material with the (211) planes accumulated at the rolled surface to improve the embrittlement crack propagation arrest characteristic has been proposed (for example, Japanese Patent Publication (A) No. 2002-24891).
The embrittlement fracture resistance characteristic is evaluated by running a drop weight tear test (referred to as a “DWTT test”) and determining the temperature at which the ductile fracture rate becomes 85% or more (referred to as the “DWTT transition temperature”). In particular, embrittlement cracks often occur from the weld zones, so it is possible to form the weld bead at the center of a test piece, introduce an embrittlement crack, and run a DWTT test for evaluation. Steel pipe excellent in this type of embrittlement fracture resistance characteristic has been proposed (for example, Japanese Patent Publication (A) No. 11-36042).
On the other hand, for the evaluation of the ductile fracture characteristic, a full crack burst test which attaches an explosive to the surface of the steel pipe, explodes it, and judges if the ductile crack formed is arrested is optimal. However, a full crack burst test is extremely high in the costs required for the test, so is being replaced by the Charpy impact test or DWTT test. This is because the results of a full crack burst test and the Charpy absorption energy or absorption energy found by the DWTT test (referred to as the “DWTT absorption energy”) match relatively well for steel with a tensile strength of up to the X70 class or so.
However, in high strength steel plate and high strength welded pipe with a tensile strength of the X100 class or more, it was learned that no correlation can be observed between the full crack burst test of steel pipe and the Charpy absorption energy and DWTT absorption energy of the material, that is, the steel plate. It was learned that the Charpy impact test and DWTT test are not suitable for evaluation of the ductile fracture characteristic of high strength steel plate. For this reason, in place of the high test cost full crack burst test of steel pipe, a test method enabling simple evaluation of the ductile fracture characteristic is required. Further, use of the findings obtained by this test so as to develop high strength steel plate and high strength welded pipe excellent in ductile fracture characteristic has been demanded.
Further, in high strength steel plate and high strength welded pipe corresponding to the X100 class, defects known as “surface parallel cracks” sometimes occur. In the present invention, “surface parallel cracks” are cracks parallel to the plate surface which in particular easily occur in the vicinity of the center of the steel plate in the plate thickness and are defects caused by hydrogen. These surface parallel cracks can be detected by ultrasonic flaw detection. High strength steel plate and high strength welded pipe are high in hydrogen-induced crack susceptibility, so sometimes surface parallel cracks are present and the ductile fracture characteristic deteriorates.